bpy.props.PointerProperty should return Optional[_T] when _T extends bpy.types.ID

[Mysteryem]

Pointer properties that are set to a bpy.types.ID are writable and may be None, an overload should be added for this case (assuming I can figure out how to so with _T being a type variable), and an overload should be added for when _T is a bpy.types.PropertyGroup, where the pointer property is read-only and never None.